Auditory specificity in unit recordings from cat's visual cortex

Triple dissociation of visual, auditory and motor processing in mouse primary visual cortex

Primary sensory cortices respond to crossmodal stimuli-for example, auditory responses are found in primary visual cortex (V1). However, it remains unclear whether these responses reflect sensory inputs or behavioral modulation through sound-evoked body movement. We address this controversy by showing that sound-evoked activity in V1 of awake mice can be dissociated into auditory and behavioral components with distinct spatiotemporal profiles. The auditory component began at approximately 27 ms, was found in superficial and deep layers and originated from auditory cortex. Sound-evoked orofacial movements correlated with V1 neural activity starting at approximately 80-100 ms and explained auditory frequency tuning. Visual, auditory and motor activity were expressed by different laminar profiles and largely segregated subsets of neuronal populations. During simultaneous audiovisual stimulation, visual representations remained dissociable from auditory-related and motor-related activity. This three-fold dissociability of auditory, motor and visual processing is central to understanding how distinct inputs to visual cortex interact to support vision.

Audio-visual experience strengthens multisensory assemblies in adult mouse visual cortex

We experience the world through multiple senses simultaneously. To better understand mechanisms of multisensory processing we ask whether inputs from two senses (auditory and visual) can interact and drive plasticity in neural-circuits of the primary visual cortex (V1). Using genetically-encoded voltage and calcium indicators, we find coincident audio-visual experience modifies both the supra and subthreshold response properties of neurons in L2/3 of mouse V1. Specifically, we find that after audio-visual pairing, a subset of multimodal neurons develops enhanced auditory responses to the paired auditory stimulus. This cross-modal plasticity persists over days and is reflected in the strengthening of small functional networks of L2/3 neurons. We find V1 processes coincident auditory and visual events by strengthening functional associations between feature specific assemblies of multimodal neurons during bouts of sensory driven co-activity, leaving a trace of multisensory experience in the cortical network.

A possible key role of vision in the development of schizophrenia

Based on a brief overview of the various aspects of schizophrenia reported by numerous studies, here we hypothesize that schizophrenia may originate (and in part be performed) from visual areas. In other words, it seems that a normal visual system or at least an evanescent visual perception may be an essential prerequisite for the development of schizophrenia as well as of various types of hallucinations. Our study focuses on auditory and visual hallucinations, as they are the most prominent features of schizophrenic hallucinations (and also the most studied types of hallucinations). Here, we evaluate the possible key role of the visual system in the development of schizophrenia.

Contextual modulation of primary visual cortex by auditory signals

Early visual cortex receives non-feedforward input from lateral and top-down connections (Muckli & Petro 2013 Curr. Opin. Neurobiol.23, 195–201. (doi:10.1016/j.conb.2013.01.020)), including long-range projections from auditory areas. Early visual cortex can code for high-level auditory information, with neural patterns representing natural sound stimulation (Vetter et al. 2014 Curr. Biol.24, 1256–1262. (doi:10.1016/j.cub.2014.04.020)). We discuss a number of questions arising from these findings. What is the adaptive function of bimodal representations in visual cortex? What type of information projects from auditory to visual cortex? What are the anatomical constraints of auditory information in V1, for example, periphery versus fovea, superficial versus deep cortical layers? Is there a putative neural mechanism we can infer from human neuroimaging data and recent theoretical accounts of cortex? We also present data showing we can read out high-level auditory information from the activation patterns of early visual cortex even when visual cortex receives simple visual stimulation, suggesting independent channels for visual and auditory signals in V1. We speculate which cellular mechanisms allow V1 to be contextually modulated by auditory input to facilitate perception, cognition and behaviour. Beyond cortical feedback that facilitates perception, we argue that there is also feedback serving counterfactual processing during imagery, dreaming and mind wandering, which is not relevant for immediate perception but for behaviour and cognition over a longer time frame.

This article is part of the themed issue ‘Auditory and visual scene analysis’.

The multisensory function of the human primary visual cortex

It has been nearly 10 years since Ghazanfar and Schroeder (2006) proposed that the neocortex is essentially multisensory in nature. However, it is only recently that sufficient and hard evidence that supports this proposal has accrued. We review evidence that activity within the human primary visual cortex plays an active role in multisensory processes and directly impacts behavioural outcome. This evidence emerges from a full pallet of human brain imaging and brain mapping methods with which multisensory processes are quantitatively assessed by taking advantage of particular strengths of each technique as well as advances in signal analyses. Several general conclusions about multisensory processes in primary visual cortex of humans are supported relatively solidly. First, haemodynamic methods (fMRI/PET) show that there is both convergence and integration occurring within primary visual cortex. Second, primary visual cortex is involved in multisensory processes during early post-stimulus stages (as revealed by EEG/ERP/ERFs as well as TMS). Third, multisensory effects in primary visual cortex directly impact behaviour and perception, as revealed by correlational (EEG/ERPs/ERFs) as well as more causal measures (TMS/tACS). While the provocative claim of Ghazanfar and Schroeder (2006) that the whole of neocortex is multisensory in function has yet to be demonstrated, this can now be considered established in the case of the human primary visual cortex.

Genomics and proteomics in solving brain complexity

The human brain is extraordinarily complex, composed of billions of neurons and trillions of synaptic connections. Neurons are organized into circuit assemblies that are modulated by specific interneurons and non-neuronal cells, such as glia and astrocytes. Data on human genome sequences predicts that each of these cells in the human brain has the potential of expressing ∼20 000 protein coding genes and tens of thousands of noncoding RNAs. A major challenge in neuroscience is to determine (1) how individual neurons and circuitry utilize this potential during development and maturation of the nervous system, and for higher brain functions such as cognition, and (2) how this potential is altered in neurological and psychiatric disorders. In this review, we will discuss how recent advances in next generation sequencing, proteomics and bioinformatics have transformed our understanding of gene expression and the functions of neural circuitry, memory storage, and disorders of cognition.

Cortical plasticity and preserved function in early blindness

The "neural Darwinism" theory predicts that when one sensory modality is lacking, as in congenital blindness, the target structures are taken over by the afferent inputs from other senses that will promote and control their functional maturation (Edelman, 1993). This view receives support from both cross-modal plasticity experiments in animal models and functional imaging studies in man, which are presented here.

Auditory influences on non-auditory cortices

Although responses to auditory stimuli have been extensively examined in the well-known regions of auditory cortex, there are numerous reports of acoustic sensitivity in cortical areas that are dominated by other sensory modalities. Whether in 'polysensory' cortex or in visual or somatosensory regions, auditory responses in non-auditory cortex have been described largely in terms of auditory processing. This review takes a different perspective that auditory responses in non-auditory cortex, either through multisensory subthreshold or bimodal processing, provide subtle but consistent expansion of the range of activity of the dominant modality within a given area. Thus, the features of these acoustic responses may have more to do with the subtle adjustment of response gain within a given non-auditory region than the encoding of their tonal properties.

Visuo-auditory interactions in the primary visual cortex of the behaving monkey: electrophysiological evidence

Background

Visual, tactile and auditory information is processed from the periphery to the cortical level through separate channels that target primary sensory cortices, from which it is further distributed to functionally specialized areas. Multisensory integration is classically assigned to higher hierarchical cortical areas, but there is growing electrophysiological evidence in man and monkey of multimodal interactions in areas thought to be unimodal, interactions that can occur at very short latencies. Such fast timing of multisensory interactions rules out the possibility of an origin in the polymodal areas mediated through back projections, but is rather in favor of heteromodal connections such as the direct projections observed in the monkey, from auditory areas (including the primary auditory cortex AI) directly to the primary visual cortex V1. Based on the existence of such AI to V1 projections, we looked for modulation of neuronal visual responses in V1 by an auditory stimulus in the awake behaving monkey.

Results

Behavioral or electrophysiological data were obtained from two behaving monkeys. One monkey was trained to maintain a passive central fixation while a peripheral visual (V) or visuo-auditory (AV) stimulus was presented. From a population of 45 V1 neurons, there was no difference in the mean latencies or strength of visual responses when comparing V and AV conditions. In a second active task, the monkey was required to orient his gaze toward the visual or visuo-auditory stimulus. From a population of 49 cells recorded during this saccadic task, we observed a significant reduction in response latencies in the visuo-auditory condition compared to the visual condition (mean 61.0 vs. 64.5 ms) only when the visual stimulus was at midlevel contrast. No effect was observed at high contrast.

Conclusion

Our data show that single neurons from a primary sensory cortex such as V1 can integrate sensory information of a different modality, a result that argues against a strict hierarchical model of multisensory integration. Multisensory interaction in V1 is, in our experiment, expressed by a significant reduction in visual response latencies specifically in suboptimal conditions and depending on the task demand. This suggests that neuronal mechanisms of multisensory integration are specific and adapted to the perceptual features of behavior.

Multisensory interplay reveals crossmodal influences on 'sensory-specific' brain regions, neural responses, and judgments

Although much traditional sensory research has studied each sensory modality in isolation, there has been a recent explosion of interest in causal interplay between different senses. Various techniques have now identified numerous multisensory convergence zones in the brain. Some convergence may arise surprisingly close to low-level sensory-specific cortex, and some direct connections may exist even between primary sensory cortices. A variety of multisensory phenomena have now been reported in which sensory-specific brain responses and perceptual judgments concerning one sense can be affected by relations with other senses. We survey recent progress in this multisensory field, foregrounding human studies against the background of invasive animal work and highlighting possible underlying mechanisms. These include rapid feedforward integration, possible thalamic influences, and/or feedback from multisensory regions to sensory-specific brain areas. Multisensory interplay is more prevalent than classic modular approaches assumed, and new methods are now available to determine the underlying circuits.

Category-specific responses to faces and objects in primate auditory cortex

Auditory and visual signals often occur together, and the two sensory channels are known to influence each other to facilitate perception. The neural basis of this integration is not well understood, although other forms of multisensory influences have been shown to occur at surprisingly early stages of processing in cortex. Primary visual cortex neurons can show frequency-tuning to auditory stimuli, and auditory cortex responds selectively to certain somatosensory stimuli, supporting the possibility that complex visual signals may modulate early stages of auditory processing. To elucidate which auditory regions, if any, are responsive to complex visual stimuli, we recorded from auditory cortex and the superior temporal sulcus while presenting visual stimuli consisting of various objects, neutral faces, and facial expressions generated during vocalization. Both objects and conspecific faces elicited robust field potential responses in auditory cortex sites, but the responses varied by category: both neutral and vocalizing faces had a highly consistent negative component (N100) followed by a broader positive component (P180) whereas object responses were more variable in time and shape, but could be discriminated consistently from the responses to faces. The face response did not vary within the face category, i.e., for expressive vs. neutral face stimuli. The presence of responses for both objects and neutral faces suggests that auditory cortex receives highly informative visual input that is not restricted to those stimuli associated with auditory components. These results reveal selectivity for complex visual stimuli in a brain region conventionally described as non-visual “unisensory” cortex.

Do cross-modal projections always result in multisensory integration?

Convergence of afferents from different sensory modalities has generally been thought to produce bimodal (and trimodal) neurons (i.e., exhibit suprathreshold excitation to more than 1 sensory modality). Consequently, studies identifying cross-modal connections assume that such convergence results in bimodal (or trimodal) neurons that produce familiar forms of multisensory integration: response enhancement or depression. The present study questioned that assumption by anatomically identifying a projection from ferret auditory to visual cortex Area 21. However, electrophysiological recording within Area 21 not only failed to identify a single bimodal neuron but also familiar forms of multisensory integration were not observed either. Instead, a small proportion of neurons (9%; 27/296) showed subthreshold multisensory integration, in which visual responses were significantly modulated by auditory inputs. Such subthreshold multisensory effects were enhanced by gamma-aminobutyric acid antagonism, whereby a majority of neurons (87%; 20/23) now participated in a significant, multisensory population effect. Thus, multisensory convergence does not de facto result in bimodal (or trimodal) neurons or the traditional forms of multisensory integration. However, the fact that unimodal neurons exhibited a subthreshold form of multisensory integration not only affirms the relationship between convergence and integration but also expands our understanding of the functional repertoire of multisensory processing itself.

Do early sensory cortices integrate cross-modal information?

Our different senses provide complementary evidence about the environment and their interaction often aids behavioral performance or alters the quality of the sensory percept. A traditional view defers the merging of sensory information to higher association cortices, and posits that a large part of the brain can be reduced into a collection of unisensory systems that can be studied in isolation. Recent studies, however, challenge this view and suggest that cross-modal interactions can already occur in areas hitherto regarded as unisensory. We review results from functional imaging and electrophysiology exemplifying cross-modal interactions that occur early during the evoked response, and at the earliest stages of sensory cortical processing. Although anatomical studies revealed several potential origins of these cross-modal influences, there is yet no clear relation between particular functional observations and specific anatomical connections. In addition, our view on sensory integration at the neuronal level is coined by many studies on subcortical model systems of sensory integration; yet, the patterns of cross-modal interaction in cortex deviate from these model systems in several ways. Consequently, future studies on cortical sensory integration need to leave the descriptive level and need to incorporate cross-modal influences into models of the organization of sensory processing. Only then will we be able to determine whether early cross-modal interactions truly merit the label sensory integration, and how they increase a sensory system's ability to scrutinize its environment and finally aid behavior.

Crossmodal audio-visual interactions in the primary visual cortex of the visually deprived cat: a physiological and anatomical study

Blind individuals often demonstrate enhanced non-visual perceptual abilities. Neuroimaging and transcranial magnetic stimulation experiments have suggested that computations carried out in the occipital cortex may underlie these enhanced somatosensory or auditory performances. Thus, cortical areas that are dedicated to the analysis of the visual scene may, in the blind, acquire the capacity to participate in other sensory processing. However, the neural substrate that underlies this transfer of function is not fully characterized. Here we studied the synaptic and anatomical basis of this phenomenon in cats that were visually deprived by dark rearing, either early visually deprived after birth (EVD), or late visually deprived after the end of the critical period (LVD); data were compared with those obtained in normally reared cats (controls). The presence of synaptic and spike responses to auditory stimulation was examined by means of intracellular recordings in area 17 and the border between areas 17 and 18. While none of the cells recorded in control and LVD cats showed responses to sound, 14% of the cells recorded in EVD cats showed both subthreshold synaptic responses and suprathreshold spike responses to auditory stimuli. Synaptic responses were of small amplitude, but well time-locked to the stimuli and had an average latency of 30+/-12ms. In an attempt to identify the origin of the inputs carrying auditory information to the visual cortex, wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was injected in the visual cortex and retrograde labeling examined in the cortex and thalamus. No significant retrograde labeling was found in auditory cortical areas. However, the proportion of neurons projecting from supragranular layers of the posteromedial and posterolateral parts of the lateral suprasylvian region to V1 was higher than that in control cats. Retrograde labeling in the lateral geniculate nucleus showed no difference in the total number of neurons between control and visually deprived cats, but there was a higher proportion of labeling in C-laminae in deprived cats. Labeled cells were not found in the medial geniculate nucleus, a thalamic relay for auditory information, in either control or visually deprived cats. Finally, immunohistochemistry of the visual cortex of deprived cats revealed a striking decrease in pavalbumin- and calretinin-positive neurons, the functional implications of which we discuss.

Involvement of the superior temporal cortex and the occipital cortex in spatial hearing: evidence from repetitive transcranial magnetic stimulation

The processing of auditory spatial information in cortical areas of the human brain outside of the primary auditory cortex remains poorly understood. Here we investigated the role of the superior temporal gyrus (STG) and the occipital cortex (OC) in spatial hearing using repetitive transcranial magnetic stimulation (rTMS). The right STG is known to be of crucial importance for visual spatial awareness, and has been suggested to be involved in auditory spatial perception. We found that rTMS of the right STG induced a systematic error in the perception of interaural time differences (a primary cue for sound localization in the azimuthal plane). This is in accordance with the recent view, based on both neurophysiological data obtained in monkeys and human neuroimaging studies, that information on sound location is processed within a dorsolateral "where" stream including the caudal STG. A similar, but opposite, auditory shift was obtained after rTMS of secondary visual areas of the right OC. Processing of auditory information in the OC has previously been shown to exist only in blind persons. Thus, the latter finding provides the first evidence of an involvement of the visual cortex in spatial hearing in sighted human subjects, and suggests a close interconnection of the neural representation of auditory and visual space. Because rTMS induced systematic shifts in auditory lateralization, but not a general deterioration, we propose that rTMS of STG or OC specifically affected neuronal circuits transforming auditory spatial coordinates in order to maintain alignment with vision.

A revised view of sensory cortical parcellation

Traditional cortical parcellation schemes have emphasized the presence of sharply defined visual, auditory, and somatosensory domains populated exclusively by modality-specific neurons (i.e., neurons responsive to sensory stimuli from a single sensory modality). However, the modality-exclusivity of this scheme has recently been challenged. Observations in a variety of species suggest that each of these domains is subject to influences from other senses. Using the cerebral cortex of the rat as a model, the present study systematically examined the capability of individual neurons in visual, auditory, and somatosensory cortex to be activated by stimuli from other senses. Within the major modality-specific domains, the incidence of inappropriate (i.e., nonmatching) and/or multisensory neurons was very low. However, at the borders between each of these domains a concentration of multisensory neurons was found whose modality profile matched the representations in neighboring cortices and that were able to integrate their cross-modal inputs to give rise to enhanced and/or depressed responses. The results of these studies are consistent with some features of both the traditional and challenging views of cortical organization, and they suggest a parcellation scheme in which modality-specific cortical domains are separated from one another by transitional multisensory zones.

Cross-modal plasticity: where and how?

Animal studies have shown that sensory deprivation in one modality can have striking effects on the development of the remaining modalities. Although recent studies of deaf and blind humans have also provided convincing behavioural, electrophysiological and neuroimaging evidence of increased capabilities and altered organization of spared modalities, there is still much debate about the identity of the brain systems that are changed and the mechanisms that mediate these changes. Plastic changes across brain systems and related behaviours vary as a function of the timing and the nature of changes in experience. This specificity must be understood in the context of differences in the maturation rates and timing of the associated critical periods, differences in patterns of transiently existing connections, and differences in molecular factors across brain systems.

Deactivation of sensory-specific cortex by cross-modal stimuli

Visual and auditory cortices traditionally have been considered to be "modality-specific." Thus, their activity has been thought to be unchanged by information in other sensory modalities. However, using functional magnetic resonance imaging (fMRI), the present experiments revealed that ongoing activity in the visual cortex could be modulated by auditory information and ongoing activity in the auditory cortex could be modulated by visual information. In both cases, this cross-modal modulation of activity took the form of deactivation. Yet, the deactivation response was not evident in either cortical area during the paired presentation of visual and auditory stimuli. These data suggest that cross-modal inhibitory processes operate within traditional modality-specific cortices and that these processes can be switched on or off in different circumstances.

Cross-modal neuroplasticity in neonatally enucleated hamsters: structure, electrophysiology and behaviour

Potential auditory compensation in neonatally bilaterally enucleated Syrian hamsters was explored anatomically, electrophysiologically and behaviourally. Gross morphology of the visual cortex appeared normal and no obvious cytoarchitectural malformation was discerned. However, enucleation induced a significant increase in the spontaneous firing rate of visual cortex cells. Further, auditory stimuli elicited field potentials and single unit responses in the visual cortex of enucleated, but not normal, animals. About 63% of the cells isolated in the visual cortex of 16 enucleated hamsters responded to at least one type of auditory stimulus. Most of the responses were less vigorous and less time-locked than those of auditory cortex cells, and thresholds were typically higher. Projection tracing with WGA-HRP disclosed reciprocal connections between the visual cortex and the dorsal lateral geniculate nucleus in both intact and enucleated animals. However, in the enucleated animals retrogradely labelled cells were also found in the inferior colliculus, the major midbrain auditory nucleus. Behaviourally determined auditory sensitivity across the hearing range did not differ between enucleated and intact hamsters. Minimum audible angle, as determined by a conditioned suppression task, ranged from around 17 to 22 degrees, with no significant difference between normal and enucleated animals. The two groups also did not differ with regard to the direction of their unconditioned head orientating response to intermittent noise. However, the enucleated animals showed a more vigorous response and were slower to habituate to the noise. These results show that bilateral enucleation of newborn hamsters results in auditory activation of visual targets, in addition to the typical activation of the intact auditory pathway. Behaviourally it appears that enucleated hamsters, compared with their normal littermates, are slower to habituate in their response to an unexpected source of sound.

Category learning through multimodality sensing

Humans and other animals learn to form complex categories without receiving a target output, or teaching signal, with each input pattern. In contrast, most computer algorithms that emulate such performance assume the brain is provided with the correct output at the neuronal level or require grossly unphysiological methods of information propagation. Natural environments do not contain explicit labeling signals, but they do contain important information in the form of temporal correlations between sensations to different sensory modalities, and humans are affected by this correlational structure (Howells, 1944; McGurk & MacDonald, 1976; MacDonald & McGurk, 1978; Zellner & Kautz, 1990; Durgin & Proffitt, 1996). In this article we describe a simple, unsupervised neural network algorithm that also uses this natural structure. Using only the co-occurring patterns of lip motion and sound signals from a human speaker, the network learns separate visual and auditory speech classifiers that perform comparably to supervised networks.

Electrophysiological evidence for cross-modal plasticity in humans with early- and late-onset blindness

It is commonly believed that sensory deprivation can lead to cross-modal reorganization in an immature but not in a mature brain. The results of the present study suggest, however, that plasticity between sensory modalities is possible even in adults: activity indicating involvement of parietal or occipital brain areas in pitch-change discrimination was found in individuals blinded after childhood. Event-related brain potentials of early blinded (before the age of 2 years), late-blinded (12-28 years of age), and sighted adults were recorded to stimulus sequences consisting of standard tones occasionally replaced by deviant tones. Even when participants were not attending to auditory stimuli, the deviant tones elicited the mismatch negativity (MMN) in each group. There were no significant MMN front-back scalp distribution differences among the groups. However, when participants were detecting deviant stimuli, these stimuli elicited N2 and P3 waves that were posterior in distribution in both groups of blind participants relative to those of the sighted participants. These results suggest that cross-modal reorganization may occur even in the mature human brain.

The spatiotemporal organization of auditory, visual, and auditory-visual evoked potentials in rat cortex

Four placements of an 8 x 8 channel microelectrode array were used to map auditory, visual, and combined auditory-visual evoked potentials (AEP, VEP, AVEP) from a total of 256 electrode sites over a 7 x 7 mm2 area including most of somatosensory, auditory, and visual cortex in the right hemisphere of the rat. The unimodal AEP and VEP consisted of an archetypal response sequence representing a systematic spatial and temporal activation of primary and secondary sensory cortex. Spatiotemporal analysis of these waveforms indicated that they could be decomposed into a small number of spatial and temporal components; components that are related to patterns of specific and non-specific thalamocortical projections connecting the auditory and visual nuclei of the thalamus with primary and secondary auditory and visual cortex. These data suggest that the AEP and VEP complex are the cortical reflection of asynchronous activation of parallel thalamocortical projection systems. The areal distribution of the AEP and VEP also overlapped, primarily in secondary auditory and visual cortex, indicating that these regions contain populations of cells responding to either modality. Polymodal auditory-visual stimulation resulted in unique activation of two isolated populations of neurons positioned in secondary auditory and secondary visual cortex which were revealed by difference waveforms, computed by subtracting the sum of the AEP and VEP from the AVEP complex. Retrograde labeling of the polymodal zones indicated that they receive parallel thalamocortical projections primarily from non-specific auditory and visual thalamic nuclei including the medial and dorsal divisions of the medial geniculate nucleus (MGm and MGd), the suprageniculate nucleus (SGN), and the lateral posterior nucleus (LP). The polymodal zone in visual cortex also receives specific projections from the dorsal division of the lateral geniculate nucleus (LGd). These data conform to a general model of thalamocortical organization in which specific thalamic nuclei with a high degree of modality specificity make restricted projections to primary sensory cortex and parts of secondary sensory cortex, and association thalamic nuclei with a high degree of sensory convergence make more divergent cortical projections. Primary and secondary sensory cortex, as well as distinct zones of polysensory cortex appear to be activated in tandem via parallel thalamocortical projections. Thus, the cerebral cortex must have simultaneous access to both unimodal and polymodal sensory information.

Ventriloquist effect reinstates responsiveness to auditory stimuli in the 'ignored' space in patients with hemispatial neglect

We examined 6 patients with robust visual neglect following right hemisphere damage. All of them had signs of auditory neglect as documented by the inferior identification of syllables delivered through a loudspeaker on the left side. When the same stimuli on the left were administered in the presence of a fictitious source of sound (a dummy loudspeaker) visible in the homolesional space, a significant increase in the identification score of sounds was obtained (the "ventriloquist" effect). The result is in keeping with a notion of a strong coupling between auditory and visual systems. The effect is attributed to the activation by the fictitious source of sound of the audio-visual map in the left hemisphere. We draw attention to the possibility that loss of awareness of auditory input may arise due to the disconnection of the visual input from the audio-visual template.

Visual and auditory cortical lesions following acquisition of an intensity discrimination in rats fail to disrupt cross-modal transfer

The effect of visual or auditory decortication on cross-modal transfer of an intensity discrimination was examined in rats. Twenty animals were first trained under either visual-auditory (V-A) or auditory-visual (A-V) cross-modal transfer (CMT) in a shuttlebox using a shock avoidance pardigm. Prior to the second training session, five of the A-V animals received auditory ablations and five V-A animals received visual ablations. The other 10 animals served as controls and received sham operations. The results reveal that CMT occurred in both experimental groups following cortical ablations. It is possible that information regarding stimulus intensity was transferred from a cortical region used during the original training session to the cortex used in the second or retraining session, prior to surgery. Alternatively, it may be that some subcortical structure (e.g. the amygdala, superior colliculus, or reticular formation) may be involved in CMT of intensity.

Development of projections from auditory to visual areas in the cat

In newborn kittens, cortical auditory areas (including AI and AII) send transitory projections to ipsi- and contralateral visual areas 17 and 18. These projections originate mainly from neurons in supragranular layers but also from a few in infragranular layers (Innocenti and Clarke: Dev. Brain Res. 14:143-148, '84; Clarke and Innocenti: J. Comp. Neurol. 251:1-22, '86). The postnatal development of these projections was studied with injections of anterograde tracers (wheat germ agglutinin-horseradish peroxidase [WGA-HRP]) in AI and AII and of retrograde tracers (WGA-HRP, fast blue, diamidino yellow, rhodamine-labeled latex beads) in areas 17 and 18. It was found that the projections are nearly completely eliminated in development, this, by the end of the first postnatal month. Until then, most of the transitory axons seem to remain confined to the white matter and the depth of layer VI; a few enter it further but do not appear to form terminal arbors. As for other transitory cortical projections the disappearance of the transitory axons seems not to involve death of their neurons of origin. In kittens older than 1 month and in normal adult cats, retrograde tracer injections restricted to, or including, areas 17 and 18 label only a few neurons in areas AI and AII. Unlike the situation in the kitten, nearly all of these are restricted to layers V and VI. A similar distribution of neurons projecting from auditory to visual areas is found in adult cats bilaterally enucleated at birth, which suggests that the postnatal elimination of the auditory-to-visual projection is independent of visual experience and more generally of information coming from the retina.

Attention to central and peripheral visual space in a movement detection task. III. Separate effects of auditory deprivation and acquisition of a visual language

We employed event-related brain potentials (ERPs) and measures of signal detectability to compare attention to peripheral and central visual stimuli in normal hearing subjects who were born to deaf parents (HD Ss) and whose first language was American Sign Language (ASL). The results were compared with those obtained from normal hearing Ss and congenitally deaf Ss in the same paradigm. Task performance and ERPs during attention to the foveal region were similar in the 3 groups. In contrast, with attention to the peripheral stimuli the deaf Ss displayed attention effects over the occipital regions of both hemispheres that were several times larger than those in the hearing and the HD Ss. However, both HD and deaf Ss displayed lateral asymmetries in behavior and ERPs that were opposite in direction to those of the hearing Ss. Whereas hearing Ss detected the direction of target motion better when it occurred in the left visual field, deaf and HD Ss performed better for right visual field targets. Consistent with these results, the amplitude of the attention-related increases in the ERPs were larger from temporal and parietal regions of the right than the left hemisphere in hearing Ss, but were larger from the left than the right hemisphere in both the HD and the deaf Ss. These results suggest that auditory deprivation and the acquisition of a visual language have marked and different effects on the development of cortical specializations in humans.

Single neuron activity related to natural vestibular stimulation in the cat's visual cortex

This study examines the possibility of a vestibular input to the visual cortex using chronically implanted cats subjected to horizontal sinusoidal rotation in the dark. In areas 17 and 18 the activity of respectively 14% and 11% of units was modified by vestibular stimulation. Both non-specific and specific influences were observed. Specific influences (42% in area 17 and 33% in area 18) were similar to the types of responses recorded in the vestibular nuclei, and were encountered more frequently within the cortex subserving the peripheral visual field. Our results could provide a neurophysiological basis for some psychophysiological observations concerning visuo-vestibular interactions.

Demonstration of a direct projection from the intralaminar central lateral nucleus to the primary visual cortex

Orthograde autoradiographic and retrograde horseradish peroxidase (HRP) tracing techniques were used to demonstrate the existence of a direct projection from the central lateral nucleus of the intralaminar complex of the thalamus to the primary visual cortex of the cat. The projection is sparser than the projections from the thalamic non-specific nuclei to other cortical areas of the cat which have been described in the past [9,20,21]. The projection to primary visual cortex is most dense in cortical layers I and VI. It does not appear to have a well-defined topographical organization within this cortical area.

The effects of changing levels of arousal on the spontaneous activity of cortical neurones. II. Relaxation and alarm

In a previous paper it has been shown that interval distributions derived from the activity of single cortical neurones can be described by log-normal curves. A cell’s temporal pattern of discharge can therefore be defined by the values of two parameters – a modal interval, and a geometric standard deviation (g. s. d.). It has also been shown that the values of both parameters change when an animal falls asleep. The modal interval becomes shorter, and the g. s. d. usually becomes larger. This paper deals with the effects of changes in arousal of animals which are awake; and, in particular, with the effects of the transition from relaxation to alarm. Single unit recordings have been made from neurones in the post-lateral and supra-sylvian gyri of unrestrained cats. In order to eliminate the direct effects of eye-movements, the experiments were carried out in complete darkness, and the animal was observed through an infrared telescope. Alarm was produced by the hiss of compressed air. An animal was said to be alarmed when he stood up abruptly and turned towards the source of the noise. Alarm produced a marked fall in the discharge frequency of those cells in post-lateral cortex which initially showed a low ( < 2 action potentials per second) rate of spontaneous activity. The discharge rate of the remaining neurones (whether in suprasylvian or post-lateral cortex) was unaffected by the sudden increase in arousal. But the temporal pattern of discharge of every cell was altered. The modal interval became longer when the animal was alarmed, and the g. s. d. usually became smaller. Such changes could have been predicted from a knowledge of the neural concomitants of the transition from sleep to wakefulness. These results suggest that the activity of all cortical neurones is affected by the level of arousal of the animal, and that this modulation takes the form of a continuum of possible modal intervals, and possible g.s.ds.

Response plasticity of lateral geniculate neurons during and after pairing of auditory and visual stimuli

Neurons of the lateral geniculate nucleus, responsive only to visual stimulation, show response plasticity during and after pairing of auditory and visual stimuli. Modal response histograms reveal a gradual decrement in the number of spikes at interstimulus pairing intervals of 0 and 100 milliseconds but not 500 milliseconds. This plasticity effect, limited to tonic units, may persist for 2 to 3 minutes after termination of click-flash pairing.